The separation of aromatic and non-aromatic hydrocarbons (generally referred to as dearomatization) from mixed hydrocarbon feeds has long been recognized as necessary and advantageous for a number of varied reasons. For example, when a BTX fraction (benzene, toluene and xylene) is the aromatic fraction it may be used as a raw material in the manufacture of petrochemicals, or as an additive for gasoline to increase its octane rating. Further, the non-aromatic fraction derived from these mixed feeds have varied uses as fuels, solvents and the like and, therefore, are also highly desirable. Such uses for the aromatic and non-aromatic fractions have resulted in the development of numerous dearomatization processes.
Other aromatic hydrocarbons include those referred to as polynuclear aromatics (PNA's) are considered to be impurities in the hydrocarbon feed and the removal of PNA's from hydrocarbon mixtures containing such materials is typically energy intensive. The removal of PNA's from hydrocarbon feedstocks such as kerosine, diesel fuel and lubricating (lube) oils may be required to improve the physical properties of the material.
Of particular interest and difficulty is the separation of the complex components present in lube oils, wherein the removal of aromatic hydrocarbons is necessary to improve the viscosity index, thermal and oxidation stability, and color of the lube oils. The presence of aromatic hydrocarbons in lube oils affects the quality of these oils due to the low viscosity index, poor thermal and oxidation stability, high carbon residue, and poor color of such aromatic hydrocarbons. The aromatic hydrocarbons present in lube oils differ significantly from the BTX fraction found in light hydrocarbon mixtures used in the production of gasoline and, as a result, present vastly different separation problems.
Various processes have been suggested for the separation of the aromatic and non-aromatic hydrocarbons of a mixed feed wherein the aromatic is a BTX fraction. Typical of these processes is a process employing an extraction column for separation of a BTX fraction wherein a selective solvent, BTX and a reflux stream is introduced to a two step distillation column. BTX is then distilled to remove water and entrained solvent. Similarly, a process has been suggested wherein two distillation columns are employed with the BTX fraction and water being distilled in the second column. In addition, a process using two distillation columns wherein the second column is employed to distill the BTX fraction and other components, has been suggested.
One goal of the prior art has related to developing a dearomatization process which lowers the cost of dearomatization. Cost reduction for dearomatization processes can be achieved by improving the selectivity of the selective solvent and by modification of the separation process scheme. U.S. Pat. No. 3,985,644 mentions one such method for modifying the process scheme and reducing dearomatization costs, i.e., by reducing the use of energy-intensive steps, e.g., distillation.
In other attempts to reduce the energy of the dearomatization a mixed hydrocarbon feed, temperature swing processes were employed with a single extraction solvent or a mixed extraction solvent. These temperature swing processes were characterized by extraction at elevated temperature followed by separation at a lower temperature. The solvents employed in these temperature swing processes exhibited the formation of an upper critical solution temperature (UCST) with solutes, such that as the temperature of the solution is raised, the solute becomes more and more soluble in the solvent. This increase in solubility continues until the UCST is reached, at which point, both phases, the solute and the solvent phases are soluble in one another, in all proportions. This phenomenon permits aromatic component of the mixed hydrocarbon feed, now concentrated in the rich solvent stream, following an extraction step to be separated by cooling the rich solvent and by fractionation to separate the aromatic extract from the solvent phase. U.S. Pat. No. 4,781,820, which is hereby incorporated by reference, illustrates this concept in a process which contacts a hydrocarbon feed containing aromatic and non-aromatic hydrocarbons with a mixed extraction solvent and water in an extraction zone to provide a rich solvent stream comprising aromatics. The rich solvent stream following the removal of the extract phase is subsequently separated by distillation to provide the dry lean solvent phase required as the feed to the extraction zone and a water phase. Others as in U.S. Pat. No. 5,022,981 have employed steam distillation, extractive distillation, and combinations thereof to obtain a separation which produces the raffinate and extract products as well as the lean solvent.
The dearomatization of lube oils is of particular interest. Dearomatized lubricating oils are, generally speaking, naphthenic- and or paraffinic-type viscous materials having a low rate of viscosity change with change in temperature, i.e., relatively high viscosity index, a high degree of thermal and oxidation stability, low carbon-forming tendency, good color, and high flash points. Lubricating oil feedstocks are generally recovered as distillates or bottoms from the vacuum distillation of crude oils. A crude lube oil fraction contains many different chemical components, e.g., paraffins, naphthenes, aromatics, and the like. In order to obtain refined lubricating oils of relatively good quality and high viscosity index, the practice has been to remove components, such as aromatic and polyaromatic compounds, which tend to lower the viscosity index of the lube oil. The removal of these aromatic components has heretofore been carried out by processes as above-described and processes such as disclosed in U.S. Pat. Nos. 2,079,885; 2,342,205; 3,600,302; 2,773,005; 3,291,728; 3,788,980; and 3,883,420.
A number of selective extraction processes are commercially practical for treating lubricating oils to improve the quality of the hydrocarbon feedstock. Typically these hydrocarbon feedstocks comprise distillate stocks having boiling points above about 288.degree. C. (550.degree. F.). Three well-known processes for such extraction of raw lube stocks are EXXON - Exol N Extraction, as described in Hydrocarbon Processing, Vol. 71, No. 11, November 1992, page 195, TEXACO - MP Refining, and TEXACO - Furfural Refining, as described in HYDROCARBON PROCESSING, Vol. 67, No. 9, September 1988, page 89. The following table contains the approximate energy consumption for a typical extraction plant processing 5,000 barrels per day of raw lube oil feedstock:
______________________________________ kJ (MMBTU/HR) ______________________________________ EXXON - EXOL N Extraction 34.2 32.4 TEXACO - MP Refining 21.3 21.2 TEXACO - Furfural Refining 32.0 30.3 ______________________________________
Processes are sought which can provide for the separation of aromatic hydrocarbons from mixtures with non-aromatic hydrocarbons by a method which is more economically advantageous or energy efficient and which overcomes the requirement for a distillation step to prepare the aromatic selective solvent for use in the extraction zone.
It is a further object of this instant invention to significantly reduce the amount of energy required to perform extraction of PNA's from lube feedstocks to improve the indices of quality in lube stocks such as viscosity index, color, etc.